- #1

- 314

- 0

Any explanations or websites would be appreciated.

You are using an out of date browser. It may not display this or other websites correctly.

You should upgrade or use an alternative browser.

You should upgrade or use an alternative browser.

- Thread starter KLscilevothma
- Start date

- #1

- 314

- 0

Any explanations or websites would be appreciated.

- #2

- 337

- 1

Sinh, cosh and tanh are the hyperbolic sine, hyperbolic cosine, and hyperbolic tangent functions. You guessed it, the h means hyperbolic.

sinh x = (e^{x}-e^{-x})/2

cosh x = (e^{x}+e^{-x})/2

tanh x = sinhx/coshx

They have a similar relation to a hyperbola as the ordinary trig. functions do to a circle.

Also:

sinh z = -i*sin(i*z)

cosh z = cos(i*z)

Where i is sqrt(-1)

sinh x = (e

cosh x = (e

tanh x = sinhx/coshx

They have a similar relation to a hyperbola as the ordinary trig. functions do to a circle.

Also:

sinh z = -i*sin(i*z)

cosh z = cos(i*z)

Where i is sqrt(-1)

Last edited:

- #3

- 314

- 0

Originally posted by Lonewolf

Also:

sinh x = -i*sin(i*z)

cosh x = cos(i*z)

Where i is sqrt(-1)

I guess z is a complex number, right?

sinh x = -i*sin(i*z) ...........(1)

sinh x = (e^{x}-e^{-x})/2...........(2)

Differentiate sin hx = e

but I don't think a function involving complex number, -i*sin(i*z), is differentiable, or am I wrong?

Why -i*sin(i*z) = e

- #4

- 337

- 1

Yes, you can differentiate functions with a complex variable, in almost the same way as a real function.

To answer, or not, the second part of your question...I don't know. Maybe someone else does.

- #5

Tom Mattson

Staff Emeritus

Science Advisor

Gold Member

- 5,549

- 8

Originally posted by KL Kam

I guess z is a complex number, right?

Lonewolf meant to use x on both sides, I think.

Differentiate sin hx = e^{x}(interesting )

No, you get:

d(sinh x)/dx=(1/2)d/dx(e

d(sinh x)/dx=(1/2)(e

d(sinh x)/dx=cosh x

but I don't think a function involving complex number, -i*sin(i*z), is differentiable, or am I wrong?

Yes, the above holds for complex variables, too.

Why -i*sin(i*z) = e^{x}-e^{-x})/2 = sin hx ? [/B]

Again, I think Lonewolf meant to put x on both sides.

- #6

- 337

- 1

#Lonewolf meant to use x on both sides, I think.

Yeah, I did. Sorry about the confusion.

- #7

- 314

- 0

oops, I swollowed the "-" sign when I differentiated e^(-x)d(sinh x)/dx=(1/2)(e^{x}+e^{-x})

Is it because we can represent a function with complex variables by using an Argand(sp?) Diagram, just as we use a Cartesian plane to represent functions with real variables?Yes, you can differentiate functions with a complex variable, in almost the same way as a real function.

(1/2)(e

Is it the defination of cos hx ? (similarly we can find the defination of sin hx)

I've heard of Euler's formula e

are the hyperbolic functions somehow related to Euler's formula?

PS. I haven't met hyperbolic functions and Euler's forumla in school's syllabus yet

- #8

Tom Mattson

Staff Emeritus

Science Advisor

Gold Member

- 5,549

- 8

Originally posted by KL Kam

Is it because we can represent a function with complex variables by using an Argand(sp?) Diagram, just as we use a Cartesian plane to represent functions with real variables?

It is because the "limit" definition of a derivative holds equally well for complex numbers.

f'(z)=lim

I think your sentence should be put the other way around:

I am probably missing some rigor there, but we have enough mathematicians here to fix me if I am wrong.

(1/2)(e^{x}+e^{-x})=cosh x

Is it the defination of cos hx ?

Yes.

I've heard of Euler's formula e^{ix}= cos x + i sin x

are the hyperbolic functions somehow related to Euler's formula?

There is an analog. You can invert the system of equations:

sinh(x)=(1/2)(e

cosh(x)=(1/2)(e

and solve for e

The result is:

e

edit: typo

- #9

Hurkyl

Staff Emeritus

Science Advisor

Gold Member

- 14,950

- 19

While it is an elementary fact of complex analysis that functions like

f(z) = -i sin (iz)

are differentiable, it is not a trivial thing to prove from scratch. One has to define complex derivatives in a manner analogous to real derivatives and derive all of the existance theorems from that.

As for the why...

Euler's identity, I think, was the first connection between complex numbers and the trigonometric functions. It states that for any positive integer n:

(cos θ + i sin θ)^{n} = cos nθ + i sin nθ

Any complex number z can be written, for some r and θ, as:

z = r (cos θ + i sin θ)

So one can write a formula for integer exponentiation:

z^{n} = r^{n} (cos nθ + i sin nθ)

This is the first inkling that the trig functions have something to do with exponents.

As to actually getting the formula for the trig functions in terms of exponentials, there are two ways to do it. One is to look at the Taylor series. The other way is to differentiate to make a differential equation and solve it. The formula for the trig functions are:

cos &theta = (1/2) (e^{iθ} + e^{-iθ})

sin &theta = (1/2i) (e^{iθ} - e^{-iθ})

The hyperbolic trig functions had been previously computed as:

cosh u = (1/2) (e^{u} + e^{-u})

sinh u = (1/2) (e^{u} - e^{-u})

So you just need to plug in the imaginary values to confirm the identities like:

cos ix = cosh x

f(z) = -i sin (iz)

are differentiable, it is not a trivial thing to prove from scratch. One has to define complex derivatives in a manner analogous to real derivatives and derive all of the existance theorems from that.

As for the why...

Euler's identity, I think, was the first connection between complex numbers and the trigonometric functions. It states that for any positive integer n:

(cos θ + i sin θ)

Any complex number z can be written, for some r and θ, as:

z = r (cos θ + i sin θ)

So one can write a formula for integer exponentiation:

z

This is the first inkling that the trig functions have something to do with exponents.

As to actually getting the formula for the trig functions in terms of exponentials, there are two ways to do it. One is to look at the Taylor series. The other way is to differentiate to make a differential equation and solve it. The formula for the trig functions are:

cos &theta = (1/2) (e

sin &theta = (1/2i) (e

The hyperbolic trig functions had been previously computed as:

cosh u = (1/2) (e

sinh u = (1/2) (e

So you just need to plug in the imaginary values to confirm the identities like:

cos ix = cosh x

Last edited:

- #10

Tom Mattson

Staff Emeritus

Science Advisor

Gold Member

- 5,549

- 8

On the home row of your keyboard, where your right pinkie finger should be, is the "semicolon" key.

It makes all the difference between &theta and θ

- #11

- 314

- 0

Θ = sin

X = A sin h θ

how can we write θ in terms of X and A ?

- #12

Hurkyl

Staff Emeritus

Science Advisor

Gold Member

- 14,950

- 19

- #13

- 337

- 1

X = A sin h θ

how can we write θ in terms of X and A ?

In almost exactly the same way. We use the inverse function sinh

So, θ = sinh

Where sinh

Share: